Power capacitor

ABSTRACT

A power capacitor (30, 40) composed of a plurality of parallel-connected strings (36) of capacitor elements (33,43), wherein each string comprises a plurality of series-connected capacitor elements, said parallel strings are interconnected at their end points only and said capacitor elements (33,43) are adapted such that, in case of a breakdown, the electrodes are welded together to achieve a solid short circuit of the faulty capacitor element. (FIG. 3)

TECHNICAL FIELD

The invention relates to a power capacitor with energy limitingproperties without fuses and wherein the design and function combinetechnical advantages of capacitors with internal fuses with thesimplicity of capacitors with external fuses.

BACKGROUND ART

Modern power capacitors are characterized by very great energy density,which makes it important to limit the consequences of a fault. Asprimary protection against capacitor explosions, fuses are used inconventional power capacitors. There are two accepted methods ofproviding power capacitors with fuses:

a) by means of internal fuses, and

b) by means of external fuses.

Protection of power capacitors by means of internal fuses isspace-demanding and renders production expensive since each capacitorelement is series-connected with a fuse. However, in relation to theconstructionally simpler protection using external fuses, protection bymeans of internal fuses entails technical advantages since a fault leadsto disconnection of the faulty element, which results in very smallcapacitance changes and increased availability of a power capacitor withinternal fuses.

Currently used power capacitors have inherent limitations which can bederived from the respective fuse technique:

Power capacitor units with internal fuses require a certain number ofparallel-connected capacitor elements in each group, which limits themaximum unit voltage to approximately 9 kV. High-voltage capacitor banksmust therefore be composed of a large number of series-connected groups.

Power capacitors with external fuses have poor protective function atcurrents higher than 30 A, which limits the maximum unit current. Inaddition, a certain number of parallel-connected capacitor units in eachgroup are required. Power capacitors protected by means of externalfuses must therefore be connected in few series-connected groups with arelatively large number of parallel-connected capacitor units in eachgroup.

Fuse-free power capacitor designed according to the invention eliminatethe above-mentioned limitations, and thus, while utilizing theinvention, power capacitors can be designed in the most economical wayin all applications.

SUMMARY OF THE INVENTION

A power capacitor, which by its design and function combines thetechnical advantages of power capacitors with an internal fuse with thesimplicity and compactness of power capacitors with an external fuse,are achieved by arranging, according to the present invention, a powercapacitor comprising a plurality of series-parallel connected capacitorelements, wherein

a) the power capacitor is composed of a plurality of parallel-connectedstrings of capacitor elements,

b) each string comprises a plurality of series-connected capacitorelements,

c) the parallel strings are interconnected only at their end points, andwherein dielectrics and electrodes included in the capacitor element arearranged such that the electrodes are welded together in case ofbreakdown and a solid short circuit without the risk of partialdischarge or restriking occurs.

Capacitor elements are preferably used in the form of so-called woundfoil capacitors of full-film type, the would foil being composed ofseveral wound turns of metal foils serving as electrodes and a soliddielectric arranged between the turns, in the form of one or morepolymer films. In case of a controlled breakdown, the metal foils arewelded together and thus achieve a solid short circuit. Such an elementmay remain in operation without the risk of restriking or partialdischarge at the fault. A similar fault in a capacitor element withmixed or paper dielectric causes partial discharge and development ofgas which in the long run, if the fault point is not disconnected bymeans of fuses, may blast away the casing surrounding the powercapacitor, However, also a breakdown in a wound foil capacitor offull-film type included in a conventionally designed power capacitorwould lead to partial discharge as a result of damage caused by theheavy energy development at the fault point of a conventionally designedpower capacitor. This is avoided by designing the power capacitoraccording to the invention such that the energy development upon abreakdown is limited.

In case of breakdown of a capacitor element, this is short-circuited andthe unit capacitance is insignificantly increased. The change incapacity in case of a breakdown is of the same order of magnitude aswhen an internal fuse disconnects a faulty capacitor element in acapacitor with internal fuses. An advantage with the design according tothe invention in relation to a power capacitor with internal fuses isthat the energy development is considerably lower, which results in aminimal risk of damage to adjacent elements or an external insulation.In addition, a power capacitor according to the invention isconsiderably simpler to install and connect during production than apower capacitor with internal fuses. An additional advantage in relationto power capacitors with internal fuses is that power capacitors builtup according to the invention are considerably more compact.

Preferably, a power capacitor according to the invention is composed Ofat least three parallel-connected strings of capacitor elements, eachstring comprising at least three capacitor elements.

In an embodiment with capacitor elements in the form of wound foilcapacitors of full-film type, where the wound foil is composed ofseveral wound turns of metal foils serving as electrodes and a soliddielectric arranged between the turns, in the form of polymer film, thewound foil capacitors are stacked one above the other and connected inseries. Two busbars are arranged along the whole stack. To thesebusbars, the strings of series-connected capacitor elements areconnected. Since the strings are connected alternately to each busbar,the polarity between adjacent strings is changed and great potentialdifferences along the capacitor stack are avoided. The maximum voltagebetween two adjacent wound foil capacitors corresponds to two elementvoltages. By this design, which is simplified in relation to knowntechnique, very compact power capacitors with energy-limiting propertiescan be designed without fuses. These power capacitors combine thetechnical advantages of internal fuses with the simplicity of powercapacitors with external fuses. The energy-limiting design also ensuresthat, on the occurrence of a fault, a controlled breakdown, that is, abreakdown with limited and controlled energy development, arises and themetal foils are welded together and bring about a solid short circuitwithout partial discharge or gas development occurring. In addition,this energy-limiting design permits the capacitor unit with the faultycapacitor element to remain in operation without the risk of restrikingor partial discharge at the fault point.

In the foregoing the invention has substantially been exemplified bypower capacitors comprising capacitor elements in the form of wound foilcapacitors of full-film type but is, of course, applicable to othertypes of capacitor elements in which a controlled breakdown leads to asolid short circuit such that the capacitor unit with the faultycapacitor element may remain in operation without the risk of restrikingor partial discharge at the fault point.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in more detail in the following withreference to the accompanying FIGS. 1-3. FIGS. 1 and 2 show powercapacitors according to the prior art with internal and external fuses,respectively. FIG. 3 shows a power capacitor without fuses according tothe invention, and FIG. 4 shows a preferred embodiment with thecapacitor elements in the form of so-called wound foil capacitors.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The power capacitor 10 shown in FIG. 1 is protected by means of internalfuses 12. The power capacitor is composed of a plurality ofseries-connected groups 11. Each group 11 comprises a plurality ofparallel-connected capacitor elements 13, each element being connectedin series with its own fuse 12. For discharge of the power capacitor, adischarge resistor 14 is arranged for each group 11 ofparallel-connected capacitor elements 13 and is connected in parallelwith the group 11. The power capacitor 10 is surrounded by a casing 15.A power capacitor according to the FIG. 1 equipped with internal fusesis complicated and expensive since it comprises a large number ofcapacitor elements 13 and fuses 12.

A power capacitor protected by means of an external fuse is shown inFIG. 2. A plurality of groups 21 of parallel-connected capacitorelements 23 are arranged in series connection. Contrary to the powercapacitor of FIG. 1, there are no fuses directly connected to thecapacitor elements 23 but the power capacitor 20 is protected by meansof an external fuse 22 arranged outside the power capacitor 20. Fordischarge of the power capacitor 20, a discharge resistor 24 isarranged. The discharge resistor 24 is connected in parallel with thechain of series-connected groups 21 of parallel-connected capacitorelements 23. However, this means that in case of a fault on an element,a high discharge energy arises at the fault point, which entails a riskof blasting of the casing 25 surrounding the power capacitor 20. Inaddition, great capacitance variations arise before and after the fusefunction.

FIG. 3 shows a power capacitor without fuses according to the invention.The power capacitor 30 is composed of a plurality of parallel-connectedstrings 36 of series-connected capacitor elements 33, the parallelstrings 36 being interconnected at their end points only. For dischargeof the power capacitor 30, a discharge resistor 34 is arranged. Thedischarge resistor 34 is connected in parallel with the strings 36 ofseries-connected capacitor elements 33. The power capacitor has nofuses, which means that in case of breakdown of a capacitor element 33,the element 33 is short-circuited and the unit capacitance isinsignificantly increased. The change of capacitance is of the sameorder of magnitude as when an internal fuse disconnects a faultycapacitor element in a capacitor unit with internal fuses. Like thepower capacitors in FIGS. 1 and 2, the power capacitor 30 is shownenclosed within a casing 35. An advantage of the design according to theinvention in relation to a power capacitor with internal fuses is thatthe energy development is considerably lower, which minimizes the riskof damage to adjoining capacitor elements or external insulation. Inaddition, a power capacitor according to the invention is considerablyeasier to install and connect during production than a power capacitorwith internal fuses. An additional advantage in relation to powercapacitors with internal fuses is that power capacitors designedaccording to the invention will be considerably more compact.

In a preferred embodiment, shown in FIG. 4, with capacitor elements inthe form of so-called wound foil capacitors 43, where the wound foil 43is built up of a plurality of wound turns of metal foils serving aselectrodes and a solid dielectric arranged between the turns, in theform of polymer film, the wound foil capacitors 43 are arranged stackedon top of each other and connected in series by means of connectiondevices 44. Two busbars 41, 42 are arranged along the entire stack 40,one busbar on each side. To these busbars 41, 42 there are connected thestrings of series-connected wound foil capacitors 43 by means of theconnections 45. The strings are connected alternately to the busbars 41,42 which are each arranged on one side, the polarity thus changingbetween adjoining strings such that great potential differences alongthe capacitor stack 40 are avoided. The maximum voltage between twoadjoining wound foil capacitors 43 corresponds to two element voltages.By this design, which is simplified in relation to the prior art, verycompact, fuse-free power capacitors with energy limiting properties maybe designed, which combine the technical advantages of power capacitorswith internal fuses with the simplicity of power capacitors withexternal fuses. The energy limiting design also ensures that, on theoccurrence of a fault, a controlled breakdown, that is, a breakdown witha limited and controlled energy development, arises, the metal foilsincluded in the wound foil capacitors 43 thus being welded together andachieving a solid short circuit without partial discharge or gasdevelopment arising. In addition, this energy limiting design permitsthe faulty capacitor element to remain in operation without the risk ofrestriking or partial discharge at the fault point.

I claim:
 1. A power capacitor, comprising:a plurality ofparallel-connected strings of capacitor elements, each string comprisinga plurality of series-connected capacitor elements, each said capacitorelements having a pair of electrodes, and each said string includingfirst and second end points, the respective first and second end pointsof each said string being respectively interconnected to form theplurality of parallel-connected strings; and said capacitor elementseach having the property that, upon an electrical breakdown of acapacitor element, the pair of electrodes of the capacitor element arewelded together to form a short circuit of the faulty capacitor element.2. A power capacitor according to claim 1, wherein the power capacitorincludes at least three of said parallel-connected strings and each saidstring includes at least three series-connected capacitor elements.
 3. Apower capacitor according to claim 1, wherein said capacitor elementsare wound foil capacitors including wound turns of metal foils formingthe electrodes and with a solid dielectric polymer film between thewound turns, said wound foil capacitors being stacked on top of oneanother, said power capacitor further comprising connection devices,connection elements and a pair of spaced busbars, said wound foilcapacitors being connected in series by said connection devices, thestack of series-connected wound foil capacitors are connected betweenthe spaced pair of busbars by said connection elements.
 4. A powercapacitor according to claim 2, wherein said capacitor elements arewound foil capacitors including wound turns of metal foils forming theelectrodes and with a solid dielectric polymer film between the woundturns, said wound foil capacitors being stacked on top of one another,said power capacitor further comprising connection devices, connectionelements and a spaced pair of busbars, said wound foil capacitors beingconnected in series by said connection devices, the stack ofseries-connected wound foil capacitors are connected between the pair ofspaced busbars by said connection elements.
 5. A power capacitoraccording to claim 4, wherein the strings of series-connected wound foilcapacitors are alternately connected to the pair of spaced busbars tochange the polarity between adjacent strings of series-connected woundfoil capacitors, thereby limiting the voltage potential difference alongthe capacitor stack.
 6. A power capacitor according to claim 3, whereinthe strings of series-connected wound foil capacitors are alternatelyconnected to the pair of spaced busbars to change the polarity betweenadjacent strings of series-connected wound foil capacitors, therebylimiting the voltage potential difference along the capacitor stack.